“My own personal feeling is that the chances of life on this planet are 100 percent...I have almost no doubt about it.” What would cause UC Santa Cruz astrophysicist Steven Vogt to make such a definitive and provocative statement?
A newly discovered planet orbiting a nearby star Gliese 581 led to the quote above. This star hosts a prolific system of at least six planets—the most recently discovered, Gliese 581g. Before discussing the particulars of the planet in question, I want to emphasize two points.
First, this discovery does not surprise me—in fact, I expected that astronomers would find planets like this as our technology continued to develop. The advances in exoplanet research over the last fifteen years are stunning—the number of planets detected (excluding the unpublished Kepler data) now exceeds 500! Not only does the exoplanet catalog grow in size, but astronomers push ever closer to detecting planets similar in size to Earth and orbiting around Sun-like stars at distances where liquid water could exist.
Second, the ability to detect planets that partially resemble Earth provides a powerful tool to test two contrasting models. The commonly assumed model, reflected in Vogt’s quote, is that life arises easily in environments that meet a rather small set of criteria. I will refer to this as the “minimalist” model. In contrast, RTB’s creation model argues that life requires a planet exhibiting numerous parameters fine-tuned to exacting specifications. Planets that meet some, but not all, of these criteria serve as test-beds to distinguish which model best describes reality. The more planets astronomers find, the more powerful tests may be conducted.
With those two points in mind, here are the particulars of the Gliese 581 system.1 The star is an M dwarf star, which means it’s smaller and less luminous than the Sun. Consequently, the habitable zone where liquid water exists resides closer to the star. Gliese 581g, a potentially habitable planet, orbits the star in 37 days at one tenth the Earth-Sun distance (for comparison, Mercury orbits the Sun in 88 days at a distance around four-tenths of the Earth-Sun distance). With such a small orbital radius, the planet quickly experiences tidal locking where the same side of the planet always faces the star.
When analyzing this system, the researchers found that the average global temperature for Gliese 581g was around 0–20 degrees Fahrenheit (although it might be higher if the greenhouse heating of the atmosphere is significantly stronger than Earth’s). However the tidal-locked condition of the planet means that the two faces of the planet experience dramatically different temperatures. On the nighttime side of the planet temperatures hover well below the freezing point, while the daytime side has temperatures near the boiling point of water. Despite these extremes, the conditions at the boundary of these two sides should experience temperatures where liquid water could exist.
Ultimately, the question remains: does this planet support life or not? We don’t know. If indeed life requires only transitory pools of liquid water then Dr. Vogt’s optimism is warranted. However, I argue that many other factors militate against this minimalist perspective. For example, this planet may have formed without enough water and it may receive too much ultraviolet radiation for life to originate or survive. Also, the thick atmosphere that Gliese 581g likely possesses may preclude life. Internal heating caused by the tidal tugs of the star also poses challenges.
One of the most potent challenges pertains to life’s origins. The minimalist model assumes that life easily arises at any location where liquid water exists. However, my colleagues at RTB argue that naturalistic scenarios for the origin of life face insurmountable hurdles.
Exciting times lay ahead in the quest to determine whether the minimalist model or the RTB model proves correct, and these newly discovered planets will play a vital role. Let the testing begin.
Other related resources of interest:
Extrasolar Planets webpage
1. Steven S. Vogt et al., “The Lick-Carnegie Exoplanet Survey: a 3.1 MÅ Planet in the Habitable Zone of the Nearby M3V Star Gliese 581,” submitted to Astrophysical Journal, https://arxiv.org/abs/1009.5733.